Bi-metal thermometers are never as accurate as the digital ones. They also have a tendency to break down and stick. Tel Tru's are the rolls-royce of Bi-metal BBQ thermometers, I concur with Ed about how good Ben's are being so close. I've got a Tel Tru waiting to be installed in the lid of one of my WSM's, the stock thermometer is well off
I tend to use the Maverick as my proper monitor at the moment as it's about the best remote probe for measuring pit temperature that's available at a reasonable price for now.
Just need someone to drop a reasonably priced thermocouple based remote pit probe onto the market
Good job on the install by the way Ben.